1. Field of Invention
The present invention relates to a three-phase brushless motor drive device and motor drive method enabling consistently starting quickly without requiring a rotor position sensor.
2. Description of Related Art
Brushless motors use a suitable number of windings in the stator winding to which current is supplied to apply a consistent amount of torque to the rotor. This requires knowing the electrical phase position of the rotor relative to the stator. Various kinds of rotor position sensors are used for knowing this relative phase position. Sensorless drive technology that does not require a rotor position sensor has also been developed due to concerns about reliability, cost, and the environment. Sensorless drive technologies generally detect the rotor position by reading the back electromotive force (back-EMF) voltage produced in the stator winding when the rotor is turning. However, because this back-EMF voltage is not produced when the rotor is not turning, various other methods of detecting the rotor position when the rotor is stopped have been proposed.
EP Patent Application Publication No. 0251785 (corresponding to Japanese Laid-open Patent Publication No. S63-69489), for example, teaches sequentially selecting the stator phase and applying a rotor position detection pulse, and detecting the rotor position from the stator phase at which the current flowing through the stator winding produces the highest amplitude.
U.S. Pat. No. 5,254,918 and No. 5350987 (corresponding to Japanese Laid-open Patent Publication No. H4-46583) sequentially select the stator phase and apply a rotor position detection pulse in the same way as EP Patent Application Publication No. 0251785. In addition, U.S. Pat. No. 5,254,918 and No. 5350987 divides the motor winding at the neutral point into a first measurement group denoting voltages near ⅓ the supply voltage and a second measurement group denoting voltages near ⅔ the supply voltage, and obtains the difference voltage between the absolute value of the minimum voltage and the absolute value of the maximum voltage for each measurement group. The difference voltages of the measurement groups are then compared and the rotor position is determined based on the energizing pattern at which the greater difference voltage is obtained.
The motor drive control circuit and motor drive device taught in U.S. Patent Application Publication No. 2004/0056628 (corresponding to Japanese Laid-open Patent Publication No. 2004-104846) are described next with reference to FIG. 36 and FIG. 37. Note that only those components required to describe the operating principle are noted below.
The three-phase motor drive device shown in FIG. 36 has a drive unit 1p, a motor 2p, and a motor drive control circuit 3p. The drive unit 1p is a three-phase drive circuit composed of n-channel MOSFET power transistors Q1p, Q2p, Q3p, Q4p, Q5p, and Q6p. The drains of power transistors Q1p to Q3p are connected to a common node that is connected to a terminal to which a drive voltage VD is applied.
The source of power transistor Q1p is connected to the drain of power transistor Q4p, the source of power transistor Q2p is connected to the drain of power transistor Q5p, and the source of power transistor Q3p is connected to the drain of power transistor Q6p. The sources of power transistors Q4p to Q6p are connected to a common node that goes to ground.
One end of motor winding Lup of the motor 2p is connected to the node connecting power transistor Q1p and power transistor Q4p, one end of motor winding Lvp of the motor 2p is connected to the node connecting power transistor Q2p and power transistor Q5p, and one end of the motor winding Lwp of the motor 2p is connected to the node connecting power transistor Q3p and power transistor Q6p. The other ends of motor windings Lup, Lvp, and Lwp are connected together.
The motor drive control circuit 3p is connected to the node connecting the drive unit 1p and the motor 2p, the common connection node of the motor windings Lup, Lvp, and Lwp, and the gates of the power transistors Q1p to Q6p in the drive unit 1p. The gates of power transistors Q1p to Q6p are controlled by drive signals D1, D2, D3, D4, D5, and D6 output from the motor drive control circuit 3p. The drive unit 1p supplies drive current to the motor 2p to turn the motor 2p. 
The motor drive control circuit 3p has a pulse generator 4p, a sequence circuit 5p, a mode selection circuit 6p, a neutral point variance detection comparator 7p, a detection level generating circuit 8p, a register 9p, a decoder 10p, a preset circuit 11p, a back-EMF voltage detection comparator 12p, a switching noise mask circuit 13p, and a drive wave generating circuit 14p. 
FIG. 37 is a waveform diagram describing the relationship between the neutral point voltage CT (y-axis) of the motor windings Lup, Lvp, and Lwp in FIG. 36 and the rotor position (x-axis) before the motor starts. According to U.S. Patent Application Publication No. 2004/0056628, the motor drive control circuit 3p supplies a rotor position detection drive signal to the drive unit 1p before the motor starts. Based on this rotor position detection drive signal, the drive unit 1p supplies a rotor position search pulse to the motor windings Lup, Lvp, and Lwp. The level of this rotor position search pulse is set so that the neutral point voltage CT varies according to the rotor position before the motor starts and the motor 2p does not turn. The motor drive control circuit 3p detects the position of the rotor before the motor starts based on this neutral point voltage CT that thus varies as shown in FIG. 37.
The detection level generating circuit 8p has a plurality of resistances each having one end connected to a node between the motor 2p and drive unit 1p and the other end connected to a common node, and shifts the level of the voltage applied to the common other ends of the resistances according to the rotor position detection drive signal.
The neutral point variance detection comparator 7p compares the output of the detection level generating circuit 8p with the neutral point voltage CT.
The motor drive control circuit 3p detects the position of the rotor before the motor starts based on the output of the neutral point variance detection comparator 7p. 
See also U.S. Patent Application Publication No. 2003/0102832 (corresponding to Japanese Laid-open Patent Publication No. 2003-174789).
Three-phase brushless motors use a wide range of winding shapes and methods of magnetizing the rotor magnet in order to structurally suppress vibration, noise, and rotational deviation.
A problem with EP Patent Application Publication No. 0251785 is that it is difficult to accurately read the peak pulse current flow when the rotor position search pulse is applied. In addition, the difference between the phases in the pulse current peak is small depending on the rotor position. This requires that there is little deviation in the electromagnetic characteristics of each phase in the stator and rotor. The technology taught in EP Patent Application Publication No. 0251785 therefore is difficult to use in inexpensive motors having insufficient phase characteristics control. Furthermore, the pulse current rises in motors in which the coil inductance is reduced for high speed performance, and the current required to achieve a desired pulse current peak difference is extreme.
U.S. Pat. No. 5,254,918 and No. 5350987 teaches technology for storing the neutral point voltage of the motor winding when the rotor position detection pulse is applied in a first measured voltage group and a second measured voltage group. The difference voltage is obtained for each group and the greater difference voltage is determined. This requires the ability to A/D convert and operate on the variation in the neutral point voltage. It is therefore to use this technology in a motor requiring stand-alone automated control or in low cost motor drive systems.
A problem with U.S. Patent Application Publication No. 2004/0056628 is that there is a range where the rotor position cannot be detected. If the motor is stopped in this range when the motor starts, it may not be possible to start the motor no matter how many times the rotor position detection pulse is applied because the motor may be stopped where the rotor position cannot be detected.
Furthermore, when the rotor position cannot be correctly detected, the combined voltage of the induction voltage and the back-EMF voltage occurs in the back-EMF voltage detection phase immediately after switching from the initial rotor position detection mode to the back-EMF voltage mode. As a result, when the rotor speed is extremely slow, the rotor position information is incorrectly detected and problems such as the rotor reversing when the motor starts may occur. Problems such as rotor reversing and loss of synchronization can thus occur even if the back-EMF voltage mode is entered from the initial rotor position detection mode when the rotor position cannot be correctly detected.
It is also difficult to use sensorless drive technologies that use variation in the neutral point voltage for initial rotor position detection for sensorless starting of motors that do not have a neutral point terminal.